The present invention relates to an engine starter system for starting an engine in response to operation of a starter switch.
On ordinary motor vehicles such as automobiles, a starter motor for starting the engine is supplied with large electric energy from a battery such a lead storage battery, which is charged by an alternator while the motor vehicle is running. As the lead storage battery is in continuous use, its internal resistance is increased and the battery is self-discharged at an increasing rate. It is known that the service life of normal lead storage batteries is about one year. When the lead storage battery in use is old, it cannot supply a large current to the starter motor at the time of starting the engine, and the battery is likely to run down. The inventor has proposed a power supply system which includes a large-capacitance capacitor that is gradually charged by the electric energy stored in a battery, irrespective of the condition of the battery, and that instantaneously discharges the stored electric energy when the engine is to be started (see Japanese Patent Application No. 63(1988)-329846).
The large-capacitance capacitor which is employed in the proposed power supply system should preferably be an electric double layer capacitor. The electric double layer capacitor has a much greater storage capacity than conventional capacitors and has a physical volume or size which is smaller than one tenth of the conventional capacitors.
If such an electric double layer capacitor is employed as a power supply for producing an instantaneous large current in an engine starter system, then the internal resistance of the electric double layer capacitor should be as small as possible. The electric double layer capacitor comprises a pair of polarized electrodes and a separator in the form of an ion exchange membrane which is interposed between the polarized electrodes. The structural details of the electric double layer capacitor are disclosed in Japanese Patent Publication No. 55(1980)-41015. If an electric double layer capacitor is employed in an engine starter system, the physical volume or size of the capacitor should be small, but its electrostatic capacitance should be as large as possible. Since the volume of a region where a paste of active carbon and an electrolytic solution is present cannot be reduced, attempts are made to make the separator thinner. If the separator is thinned, more electrons pass through the separator. Therefore, with the electric double layer capacitor connected parallel to a battery at all times, a current discharged from the battery always flows through the electric double layer capacitor, with the result that the battery tends to run down soon.